摘要： To realize high efficiency in parallel with low cost, a light‐trapping rear texture was proposed to be implemented in substrate‐free thin‐film multijunction (MJ) cells. A detailed‐balance theory was formulated taking account of the finite light absorption in thin subcells. Such presented absorption model is general and useful to optimize the subcell thickness for MJ solar cells with light‐trapping design. It is applied for InGaP/GaAs/InGaAs triple‐junction solar cells to simulate subcell photocurrents and to obtain the current‐matching (minimum requisite) subcell thicknesses combinations. Furthermore, the detailed‐balance conversion efficiency was estimated for both radiative limit and the cases with below‐unity internal radiative efficiency. For InGaP/GaAs/InGaAs MJ cells with InGaP subcell thickness less than 600 nm, adding a random‐textured rear reflector can enhance light absorption so significantly that over 90% of InGaAs‐cell thickness and even 50% of GaAs‐cell thickness would be cut without any penalty in conversion efficiency, compared with the subcell thicknesses in traditional MJ cells with flat rear reflectors. Additionally, the thickness combination, (InGaP, GaAs, and InGaAs) = (450 nm, 333 nm, and 26 nm), is recommended to achieve both high conversion efficiency and low material cost. This work provides a very important theoretical guidance for the development on low‐cost and high‐efficiency MJ devices.